A concept of → star formation
in which → high-mass stars and
→ low-mass stars form in different physical conditions
involving different → molecular clouds.
Following the pioneering suggestion of Herbig (1962), successive investigations
have generally supported the idea that star formation proceeds bimodally with
respect to stellar mass. The star formation rate appears to differ both spatially
and temporally for low mass and → massive stars.
This is of considerable importance
for galactic evolution, since the low-mass stars lock up mass and are long-lived,
low luminosity survivors to the present epoch, whereas massive stars are short-lived,
recycle and enrich interstellar gas, and leave dark remnants while producing
a high luminosity per unit of mass
(Silk, J., 1988, in Galactic and Extragalactic Star Formation, p. 503, eds.
R. E. Pudritz and M. Fich).

The division of galaxies into a "red sequence" and a "blue sequence"
in the → color-magnitude diagrams of galaxies
involving large statistical surveys.
In both sequences, redder galaxies tend
to be brighter. The blue sequence is truncated at the red magnitude ~ -22,
while the red sequence
extends to brighter magnitudes. The division between the two classes
of galaxies is associated with a critical stellar mass ~
3 × 1010Msun.
Galaxies below the critical mass are typically blue, star forming
spirals and reside in the field. Galaxies above the critical mass are dominated
by red spheroids of old stars and live in dense environments
(Kauffmann et al, 2003, MNRAS 341, 33 & 54).